Ultrasonic transducers are used in many applications to produce and sense mechanical vibrations in the ultrasonic frequency range. In a number of these applications, it is desirable to use a transducer that has a specific beam pattern. For example, if the transducer is used to monitor the flow of fluid, the beam pattern should define an angle of less than 90 degrees with respect to the direction of fluid flow to ensure a suitable transducer output.
In many instances, it is also desirable for the transducer to be relatively small or have a low profile. For example, a low profile may be necessary to allow the transducer to be introduced into a confined vessel or environment and to reduce the disruptive effect of the transducer on fluid flowing in the vessel.
One particular application of interest for such transducers is the determination of volumetric flow in an intravascular conduit. In that regard, catheter-based ultrasound systems have been developed to determine a patient's cardiac output, i.e., the volumetric flow rate of blood in the patient's pulmonary artery. Such systems employ a transducer positioned close to the distal end of a catheter. This transducer is connected to a termination assembly at the proximal end of the catheter by electrical wires threaded through one or more of the catheter lumens. A bedside monitor attached to the termination assembly applies a high-frequency electrical signal (typically in the megahertz range) to the transducer, causing it to emit ultrasonic energy. Some of the emitted ultrasonic energy is then reflected by the blood cells flowing past the catheter and returned to the transducer. This reflected and returned energy is shifted in frequency in accordance with the Doppler phenomenon.
The transducer converts the Doppler-shifted, returned ultrasonic energy to an output electrical signal. This output electrical signal is then received by the bedside monitor via the lumen wiring and is used to quantitatively detect the amplitude and frequency-shifted Doppler signal associated with the ultrasonic energ reflected from the moving blood cells.
The shifted frequency of the reflected and returned energy is proportional to the cosine of the angle between the ultrasonic beam and the direction of blood flow. Thus, if the angle between the ultrasonic beam and direction of blood flow is 90 degrees, there will be no shift in frequency and, hence, no Doppler output signal. As a result, the ultrasonic beam must be launched at an angle of less than 90 degrees with respect to the blood flow.
Existing ultrasonic measurement systems process the amplitude and frequency shift information electronically to estimate the average velocity of the blood flowing through the conduit in which the transducer-carrying catheter is inserted. Such systems also require that an independent estimation of the cross-sectional area of the conduit be made using one of a variety of techniques taught in the literature, including, for example, the approach disclosed in U.S. Pat. No. 4,802,490. Cardiac output is then computed by multiplying the average velocity and cross-sectional area estimates.
As will be appreciated, the ultrasonic transducer used in such an intra-vascular application must be sufficiently small to positioned in the intravascular conduit. In addition, the transducer should emit ultrasonic energy at an angle of less than 90 degrees with respect to the direction of blood flow. Further, the surface of the transducer from which ultrasonic energy is emitted and received should not disrupt the blood flow, to avoid affecting the cardiac output determination. In view of these observations, it would be desirable to provide an ultrasonic tranducer having a low profile, for use in limited spaces and to achieve minimal flow disruption.